The invention relates to a multistage telescopic boom, in particular for a loading crane on a truck, where a hydraulic unit, comprising piston and cylinder, is arranged between each pair of successive, telescopic arms, where at least at the cylinder of an innermost arm a pipe extends axially from the cylinder bottom into the cylinder chamber. Such pipe reaches into a hollow interior of the piston rod so as to be sealed with respect to the piston when the piston is at least partially driven in. Thus, the cylinder chamber is sealed with respect to the interior of the hollow piston rod.
Sequence controls for loading cranes have the purpose of ensuring that the individual arms of a telescopic boom are telescoped or retracted in a specific, usually fixed order of sequence. The loading cranes used in the past by the company Palfinger Aktiengesellschaft are equipped with a mechanically simple sequence control of the above type, but which has, however, the drawback that only that arm of the telescopic boom that is braced directly against a folding arm enveloping such that arm is automatically telescoped first. The other arms of the telescopic boom (thrust arm) are, in contrast, not fixed with respect to the order of sequence in which they are telescoped. In the case of the cranes of Palfinger Aktiengesellschaft in prior public use, a valve which is actuated by means of a stop attached to the folding arm is on the piston side end of the hydraulic cylinder that actuates the first arm, such end being directed toward the folding arm. Not until this first arm is totally telescoped does the valve, designed as a non-return valve, allow the hydraulic fluid to flow through to the cylinders of the other telescope arms.
DE-A 34 13 443 shows a three stage telescopic boom with a controller to determine any arbitrary order of telescoping sequence. The hydraulic oil is fed into a central cylinder by way of a telescopic sliding line which is attached to a base cylinder and which is complicated and occupies a lot of space on the side next to the piston rod of the base cylinder. The hydraulic oil is fed into an outermost cylinder by way of a pipe which projects axially from the cylinder bottom and which extends so as to seal with respect to the piston into a hollow piston rod connected to the controller. The oil flowing out of the pipe of the base cylinder flows by way of a line into another pipe which projects from the bottom of the central cylinder and leads sealingly with respect to the piston in such cylinder into the hollow piston rod. The end of this hollow piston rod communicates with the cylinder chamber of the outermost hydraulic unit. In the device of DE-A 34 13 433 the order of sequence of the telescoping process is controlled exclusively by way of the external controller, which can control any arbitrary sequence of telescoping. Yet it is not ensured that the telescopic arms are telescoped automatically starting from the innermost arm. In addition, DE-A 34 13 443 does not teach how more than three hydraulic units are possible for a telescopic boom with more than three stages.